Organic Matter and Silicon in Relation to the Crystallinity of Soil Iron Oxides

The contents of pyrophosphate-extractable carbon and iron, EDTA-extractable iron, dithionite-EDTA-extractable iron and silicon, total carbon, and sodium hydroxide-extractable silicon were determined in thirty soil samples from Denmark and Tanzania in order to investigate the general influence of organic matter and silicon on the crystallinity of soil iron oxides. The ratio between the contents of EDTA-extractable iron and dithionite-EDTA-extractable iron, both corrected for pyrophosphate-extractable iron, was used as a measure of the crystallinity of the iron oxides. A close correlation (p<0.01) was found between this ratio and pyrophosphate-extractable carbon. Negative correlations were found between the ratio and the two silicon fractions. A positive highly significant (p<0.001) correlation coefficient was, however, found between the crystallinity of the iron oxides and their silicon content, taken as the difference between dithionite-EDTA-extractable silicon and sodium hydroxide-extractable silicon, corrected for dilution. The influence of organic matter and silicon on the crystallinity of soil iron oxides was further substantiated by multiple regression analysis showing that 60% of the variation in iron oxide crystallinity could be attributed to fractions of organic matter and silicon. Despite this general trend, a substantial part of the variation in iron oxide crystallinity is due to other factors and the correlations for some soil groups are inconclusive.